Sub-surface safety valves (SSSV) are used in production tubing to control the well and to close it off to prevent a blowout. Typically, these valves have a disc shaped valve member that is known as a flapper. The flapper pivots over 90 degrees between an open and a closed position. A shiftable tube known as a flow tube is movable between two positions. When shifted down it engages the flapper to rotate it 90 degrees and keeps advancing as the flapper is moved into a position behind the flow tube. In this position the SSSV is open. A closure spring which was compressed as the flow tube opened the SSSV is used to return the flow tube to the original position. When the flow tube rises a pivot spring on the flapper urges it up against a seal surface to close off the production tubing.
Typically, a control line is run adjacent the production tubing from the surface to a piston bore in the SSSV. There are several types of pistons that can be used and they are generally linked to the flow tube such that applied and retained pressure in the control line acts on a piston that is linked to the flow tube to hold the flow tube down against a closure spring and keep the flapper in the open position. One common piston type is a rod piston called that because of its shape. Other piston types can have an annular shape. The rod piston sits in an elongated bore in a main housing component of the SSSV that usually terminates in a two step male thread also known as a pin. The pin is made up to a female thread called a box to fully assemble the SSSV.
More recently demand has been for SSSVs that have higher and higher internal working pressure ratings. These demanded working pressures have gone as high as 20,000-30,000 PSI. Testing of current designs under these conditions revealed that they could comfortably hold such working pressures but the presence of the piston bore in the pin part of the housing connection experienced dimensional distortion, generally becoming asymmetrical. The reason for this is that the pin is thinner than the box in the thread area. When the pressures get high enough, the pin deflects until a clearance comes out of the two step thread, at which time the pin and box move together. Thus, the problem that is addressed by the present invention is defined as how to keep the piston bore from distorting under high loads. Two approaches are presented. One involves a sleeve inserted into the piston bore so that bore distortions become irrelevant to the continuing ability of the piston to seal because the sleeve does not distort at all or to the point where a pressure seal around the piston is lost. Another approach is the creation of parallel bores to the piston bore so as to make the pin wall more uniform in strength in the vicinity of the piston bore to hold down or eliminate the distortion in the piston bore under loading to the point where the piston seal holds and the flow tube can continue to be powered down against a closure spring. These and other aspects of the present invention will become more apparent to those skilled in the art from a review of the preferred embodiment that is described below along with its associated drawings, recognizing that the full scope of the invention is to be found in the appended claims.
Injection bores in SSSVs have been used to deliver chemicals behind the flow tube as illustrated in U.S. Pat. No. 6,148,920 and US published application US 2005/0098210. Also relevant to SSSV in general are U.S. Pat. Nos. 4,042,023; 4,399,871; 4,562,854; 4,565,215; 5,718,289 and 6,148,920 and US application 2004/0040718.